1. Field of the Invention
The present invention relates to an apparatus for iteratively estimating an intra-arterial blood-pressure value of a living subject, based on information non-invasively obtained from a circulatory system of the subject.
2. Related Art Statement
There is known an automatic blood-pressure ("BP") measuring device which non-invasively measures an intra-arterial BP value of a living subject, such as a "Korotkoff-sound-type" BP measuring device or an "oscillometric-type" BP measuring device. The Korotkoff-sound-type BP measuring device determines a BP value of a subject based on the first and/or last detection of Korotkoff sounds produced when a pressing pressure of a pressing belt wrapped around a body portion of the subject is changed, and the oscillometric-type BP measuring device determines a BP value of a subject based on variation of respective amplitudes of heartbeat-synchronous pulses produced when a pressing pressure of a pressing belt is changed.
Meanwhile, in an operation room or an intensive care unit, it may be required to periodically measure a BP value of a patient at the shortest possible period, so that a medical staff can give one or more urgent treatments to the patient. However, in the above-indicated prior BP measuring device, no BP values can be obtained before several tens of seconds have passed from the commencement of operation of the measuring device. In the case where a BP value of a patient is periodically measured at a very short period, the body portion of the patient around which the pressing belt is wrapped may suffer congestion due to the frequency compression of the pressing belt, and the obtained BP values may include errors due to the congestion of the patient.
In this background, there has been proposed a non-invasive and continuous BP estimation device which calculates, based on a signal non-invasively obtained from a living subject, a velocity at which a pulse wave propagates through an arterial vessel of the subject, and continuously estimates a BP value of the subject based on the calculated velocity according to a predetermined relationship. This BP estimation device is disclosed in, e.g., Japanese Utility Model Application laid open for inspection purposes under Publication No. 7(1995)-9305 and Japanese Patent Application laid open for inspection purposes under Publication No. 7-308295.
However, the above-identified BP estimation device estimates a BP value based on only a pulse-wave propagation velocity or a pulse-wave propagation time, and accordingly cannot enjoy a satisfactory accuracy of estimation of BP values. Thus, the prior BP estimation device needs to frequently calibrate itself based on one or more BP values measured by a Korotkoff-sound-type or oscillometric automatic BP measuring device using a pressing belt.